1. Field of the Invention
The present invention relates to a scramble system for use in either a recording and reproducing system such as a digital video recorder or the like, or a transmission and receiving system for use through communication line such as an optical, coaxial or wireless communication line, in particular, to a scramble system comprising a scramble apparatus for scrambling a digital video signal and a descramble apparatus for descrambling the scrambled digital video signal, wherein, in further particular, the scrambled digital video signal is transmitted through an optical, wire or wireless communication line, or the scrambled digital video signal is recorded into a recording medium such as a magnetic tape, a magnetic disk, a magneto-optical disk, a compact disk or the like.
2. Description of the Related Art
A conventional scramble and descramble system including a scramble apparatus and a descramble apparatus is disclosed in, Lebrat, "Method and Device for Scrambling-unscrambling Digital Image Data", U.S. Pat. No. 5,208,857, as issued on May 4, 1993 (referred to as a first reference document 1 hereinafter). The conventional scramble and descramble system is used in a digital video recording or transmission system, in which a video signal is compressed according to the DCT (Discrete Cosine Transform) compression method in the unit of a DCT block composed of a plurality of pixels, and then the compressed video signal is recorded onto a magnetic recording medium such as a magnetic tape or disk or is transmitted through a communication line.
In the conventional scramble and descramble apparatus, a video signal prior to the compression process is subjected to a scramble process by replacing DCT blocks thereof, while the compressed and decompressed video signal is subjected to a descramble process by inversely replacing the DCT blocks thereof. When performing the scramble process, the video signal located in each DCT block does not change, and therefore, the video image can be scrambled without any influence on the compression efficiency.
Further, there has been proposed a digital video recording and reproducing system as disclosed in C. Yamamitsu et al., "A Study on Trick Plays for Digital VCR", IEEE Transactions on Consumer Electronics, Vol. 37, No. 3, August 1991 (referred to as a second reference document hereinafter). The conventional digital video recording and reproducing system performs not only a process for interpolating an error in reproducing a recorded image signal but also a high speed playback process in reproducing the recorded video signal.
When the above-mentioned former conventional scramble and descramble system is applied to the above-mentioned latter conventional digital video recording system for executing he interpolation process and the high speed playback process, the following problems occurs.
Each of the interpolation process and the high speed playback process in the digital video recording and reproducing system is performed in the unit of SYNC block composed of a plurality of DCT blocks, wherein the SYNC block will be described later in detail in the detailed description of the preferred embodiments. The above-mentioned interpolation process includes replacing a SYNC block having an error with the SYNC block located in the same position of the previous frame, while the above-mentioned high speed playback process includes producing a video signal of one frame by collecting the SYNC blocks from a plurality of frames without changing the position of the SYNC blocks in each frame.
As described above, the above-mentioned interpolation and high speed playback processes are provided for processing a video signal composed of a plurality of frames, and each thereof is a process for producing a further video signal by moving the SYNC block between frames or thinning out SYNC blocks, without changing the position of the SYNC blocks in each frame. In other words, each of the above-mentioned interpolation and high speed playback processes includes a process for producing a further video signal using the SYNC blocks of the original video signal.
In the specification, this kind type of process is referred to as "an image reconstruction process" using the SYNC blocks. The same process as described above can or may be executed using the fundamental video blocks other than the SYNC blocks. This type of process is referred to as an image reconstruction process using the fundamental video blocks.
When the image reconstruction process using the SYNC blocks such as the interpolation process, the high speed playback process or the like is executed in a stage between the scramble process and the descramble process of the conventional scramble system, a problem occurs as follows.
In a video signal that has been subjected to the image reconstruction process using the SYNC blocks after changing the positions of DCT blocks thereof in a scramble process, there exists a mixture frame composed of a plurality of SYNC blocks of the scrambled video signal. Subsequently, the DCT blocks are restored to the original positions through reverse replacement of the replacement process executed in the scramble process. However, in regard to the mixture frame, one frame of a plurality of original frames is to be descrambled. Therefore, the SYNC blocks brought from the other frames cannot always be restored to the original position. When the image reconstruction process using the SYNC blocks is executed in a stage between the scramble process and the descramble process, it is not always achieved to correctly execute the descramble process.
For instance, in the case of the interpolation process, one SYNC block of the current frame is replaced by the SYNC block of the previous frame. In the SYNC block, the DCT block prior to the interpolation process and the DCT block after the interpolation process are not always placed in the same position prior to the scramble process in a frame. In the descramble process, the processing is executed so that a DCT block of a SYNC block prior to the interpolation process is restored to the original position. Therefore, the DCT block in the SYNC block after the interpolation process is not always restored to the correct position.
As described above, the conventional scramble system has such a problem that the descramble process cannot always be executed correctly when the system is employed in the digital video recording and reproducing system for executing the interpolation process and/or the high speed playback process.
Further, for example, when the scramble process is effected on a video signal before the signal is transmitted into a digital video recording and reproducing system for executing a compression process according to a adaptive quantizing method in the conventional scramble system, the compression efficiency changes so as to result in such a problem than the resulting reproduction image possibly deteriorates as compared with the original image.